"a great blow": This tells me only of a great blow (as being forbidden). Whence do I derive (the same for) a negligible one? From ("lest he smite him more) than these." If so, why is it written "a great blow"? To teach that the first ones are great blows (i.e., given with all his strength.)
"and he be demeaned" — whence they ruled that if he was soiled either by excrement or urine (while being smitten), he is exempt (from the remaining stripes). R. Yehudah said: a man, by excrement; a woman, by urine.